This invention relates generally to electrode-type glow discharge devices which can be used for sputter cleaning and bias sputtering. More particularly, the invention relates to such devices of a type utilizing a magnetic plasma trap for more efficient sputtering.
The process of cathode sputtering is well known, and has been described in many publications and patents, some of which are listed in U.S. Pat. No. 3,884,793, entitled "Electrode Type Glow Discharge Apparatus" and issued in the names of Alan S. Penfold and John A. Thornton. In essence, the cathode sputtering process operates to remove atoms from a target electrode with sufficient energy that they can interact with the atomic structure of a work surface, usually referred to as a substrate, and form thereon a permanent coating of the material removed from the target electrode. In a diode sputtering apparatus, the target electrode is connected as a cathode, and is placed in a discharge chamber to provide a low pressure gaseous environment which becomes ionized in the vicinity of the target cathode. Ionized atoms of the gas bombard the target cathode and drive off, that is sputter, atoms of the target material. The substrate to be coated with the target material is positioned in the path of the sputtered atoms, which then recombine on the substrate surface to form a coating having generally the same chemical composition as the target material, although not necessarily the same physical properties.
In sputter cleaning, the substrate itself is connected as a cathode, and becomes the target for bombardment by ions of the gas. The sputtered atoms of substrate material, and of any surface contaminants, may be collected on an appropriately located shield. After the substrate has been cleaned by this process of atomic removal, a target cathode may be activated, so that the target material is then coated onto the sputter cleaned substrate surface.
In bias sputtering, there are two target cathodes, one being of the material to be applied as a coating, and the other being the substrate itself, which is usually biased to a negative potential of lesser magnitude than that of the target cathode of coating material. While atoms are being removed from the substrate by sputtering, target material atoms are simultaneously applied as a coating to the substrate, at a greater rate than the rate of substrate atom removal. The transition from sputter cleaning to bias sputtering, or from sputter cleaning to sputter coating, may be made in such a way that, during the transition, sputtered material is passed back and forth between the target and substrate cathodes. Accordingly, an interface of graded composition can be formed between the substrate and the resultant coating. Bias sputtering is utilized in some instances to modify the metallurgical structure of the substrate coating and, in some instances, to remove impurity atoms which become weakly bonded to the surface of the growing coating during deposition, and which might otherwise become incorporated into the coating.
It is highly desirable in cathode sputtering processes for the ionized gas, i.e. the gas plasma, to be confined to a region close to the cathode. The ions produced in the plasma will then most likely be drawn to the cathode, and not lost to the walls of the surrounding discharge chamber. The aforementioned patent is directed to apparatus for maintaining one or more plasma traps in the vicinity of a cathode. The plasma is trapped by an appropriately shaped magnetic field which also has the effect of inducing electrons in the plasma to follow a spiral path, thereby encouraging a relatively large number of collisions with neutral gas atoms, and producing ions at a desirable high rate.
In accordance with the teachings of the aforementioned patent, the cathode takes the form either of a cylinder or post with outwardly projecting end flanges, wherein the plasma is confined to a cylindrical sheet around the exterior of the cathode, or of a hollow cylinder having inwardly projecting end flanges, wherein the plasma is confined to an internal cylindrical sheet. In either case, the substrate to be coated is normally positioned outside of the plasma region. Consequently, sputter cleaning and bias sputtering cannot be conveniently effected since there is little plasma in the vicinity of the substrates. In cathode sputtering devices in which no attempt is made to confine the plasma close to the cathode, i.e., in which there are no plasma traps, sputter cleaning or bias sputtering can be accomplished merely by applying the appropriate negative potential to the substrates, which would then become targets for ions in the unconfined plasma cloud. These devices, however, suffer from the disadvantages pertaining to sputtering devices which have no plasma traps to increase sputtering efficiency.
Accordingly, one result of the substantial improvements in cathode sputtering processes taught in the aforementioned patent is that there is now a clear need to provide a technique for sputter cleaning and bias sputtering in devices which utilize plasma traps to confine the ionized gas in proximity to the cathode. In general, substrates are not cylindrical in shape, and do not therefore lend themselves to sputter cleaning by techniques which utilize plasma trap configurations. Furthermore, when attempting to perform bias sputtering for purposes of modifying the metallurgical structure it is necessary that the ion current to the substrate be sufficient to re-sputter a significant fraction of the coating flux. Very high coating fluxes are possible with the teachings of the aforementioned patent. Thus, high bias sputtering rates are required to affect the metallurgical structure. Such bias sputtering rates cannot conveniently be achieved with most substrate shapes because of their low sputtering efficiency. The present invention is directed to a solution to these problems.